Currently, most commercial audio equipment has stereophonic (stereo) sound playback capability. Stereo sound provides a more natural and pleasant quality than monaural (mono) sound. Nevertheless there are still some situations which employ mono sound signals including telephone conversations, TV programs, old recordings, radios, and so forth. Stereo synthesis creates artificial stereo sounds from plain mono sounds attempting to reproduce a more natural and pleasant quality.
The present inventors have previously described two distinctively different synthesis algorithms. The first of these U.S. patent application Ser. No. 11/560,397 entitled BAND-SELECTABLE STEREO SYNTHESIZER USING COMPLEMENTARY FILTER PAIR applies comb filters [referred to in the disclosure as complementary linear phase FIR filters] to a selected range of frequencies. Comb filters are commonly used in signal processing. The basic comb filter includes a network producing a delayed version of the incoming signal and a summing function that combines the un-delayed version with the delayed version causing phase cancellations in the output and a spectrum that resembles a comb. Stated another way, the composite output spectrum has notches in amplitude at selected frequencies. When arranging separate comb filters to produce allocated notches of at different frequencies for left and right channels, the outputs from the both channels become uncorrelated. This causes the band-selected sound image to be ambiguous and thus wider. Typically, the purpose of band selection is to centralize just the human voices. The second earlier invention U.S. patent application Ser. No. 11/560,397 entitled LOW COMPUTATION MONO TO STEREO CONVERSION USING INTRA-AURAL DIFFERENCES describes the use of an Intra-Aural Time Difference (ITD) and an Intra-Aural Intensity Difference (IID). This simulates the cultural fact that, in many live orchestras and some rock bands, the low instruments tend to be located toward the right and the high instruments on the left. To do this, the incoming mono signal is split into three frequency bands and then sent to left and right channels with different delays and gains for each channel, so that the band signals add up to the original, but with ITD and IID in low and high bands respectively.
FIG. 1 illustrates a functional block diagram of a stereo synthesis circuit using intra-aural time difference (ITD) and an intra-aural intensity difference (IID). The input monaural sound 100 is split into three frequency ranges using high pass filter 101, mid-band pass filter 102 and low pass filter 103. Mid-band frequencies 119 are passed through sample delayA 104 and sample delayD 107. High pass frequencies 121 are passed to sample delayB 105 and low pass frequencies 124 are passed to sample delayC 106. The output of sample delayB 105 supplies the input of high band attenuation 108 which forms signal 123. The output of sample delayC 106 supplies the input of low band 109 which forms signal 126. The resulting six signal components 121 through 126 are routed to two summing networks 110 and 111. Summing network 110 combines high pass output 121, mid-band delayed output 122 and low pass delayed and attenuated output 126. The resulting left channel signal 116 is amplified by left amplifier 112 and passes to left output driver 114. In similar fashion, summing network 111 combines low pass output 124, mid-band delayed output 125 and high pass delayed and attenuated output 123. The resulting right channel signal 117 is amplified by right amplifier 113 and passes to right output driver 115.